You have unwittingly described why ID is not falsifiable. It starts with the premise that there is no possible path to the flagellum by small steps. When a possible path is presented, suddenly the criterion is: no speculation; show us the actual path.
When the flagellum was first proposed as irreducible, it was found to be have subcomponents that actually exist in living bacteria. When this was pointed out, ID advocates came up with other excuses. The gaps are getting smaller, along with the god people insist on finding in them.
You mean using logical symbols such as A,B,C,D?. When you say possible do you mean merely conceivable, or merely logically possible? Point to any scientific article that demonstrates an ontologically possible path, and not just a metabolic pathway, but the required assemblage of parts. (The flagellum is not just a metabolic pathway.) If that happens, a design hypothesis for this nano outboard motor would be finished off, or at least rendered superfluous.
It does seem though that it is effectively impossible to falsify the Darwinian claim if that involves showing that there is NO conceivable Darwinian pathway by which a flagellum could have evolved, which would require proving a universal negative.
Cordially,